In a cell-structured radiomobile system, in which the different available frequencies are used at the same time in different cells, there is the so-called problem of isofrequency (co-channel) interference, that is the simultaneous presence of a useful desired signal and of an undesired interfering signal having the same frequency of the useful signal, and the same type of modulation as well, with a level such as to considerably disturb the reception of the useful signal.
The technical problem at the basis of this invention is therefore to eliminate or at least to reduce within acceptable limits, the effect of an isofrequency interfering signal in a digital radio transmission system with signal phase modulation. In almost all the cases the kind of modulation of the interfering signal is known, being in practice a signal "within" the system, and that for another area of the system it is even a useful signal, and just for this reason it presents all the presuppositions to be added to the useful signal of the subject cell.
Different techniques are known aiming to reduce or eliminate an isofrequency interfering signal.
One of these techniques, particularly developed for military utilizations against jamming interferences, foresees the use of one or more auxiliary antenna for the reception of the sole interfering signal, or of the useful signal plus the interfering one, and processing of the different signals received to eliminate the interfering signal.
In some cases antenna arrays with gain and phase shift (fixed or adaptive) which determine a directivity "notch" in the direction from which the signal comes, are employed.
This technique requires the use of several suitably configurated antennas and the knowledge of the direction the interfering signal comes from being therefore very complex.
Another known technique is applied to angular modulated signals (and therefore at constant envelope). This technique is based on the principle that the presence of an isofrequency interfering signal, added to the useful signal, changes the envelope of the whole signal. Envelope fluctuations enable elimination of the element due to the interfering signal on the demodulated one. This technique is particularly applied in radio links connection between fixed points. However, in radiomobile transmission systems, the almost constant presence of multiple echos at the receiver input results in fluctuations of the envelope of the signal received, making difficult the separation of the effect of the possible isofrequency interfering signal from that of echos on the useful signal.
Finally, Phase Locked Loop or PLL receivers are known and employed for the demodulation of frequency modulation analogue signals, with partial suppression of the isofrequency interfering signal, but which cannot be used nor adapted to digital information transmission systems.
An Interference Canceller is reported in the "1989 International Conference on Acoustics, Speech and Signal Processing, vol. 2, May 1989, New York, pages 1392-1395". The proposal refers to a continuous transmission system, where a loop adaptation of receiver parameters can be implemented, with acceptable transient times. Moreover, Maximum Likelihood Estimation is used not as the primary estimation process, but just in the context of an error correction process. Finally, the error detection procedure, based on the examination of a difference signal after signal reconstruction, works only under low BER (bit error rate) conditions, where the contribution of a single errored bit does not overlap with other errored bits.
Loop adaptation of receiver parameters is not considered in a TDMA, or in general in a burst mode transmission system since the burst length is in general too short to allow parameter setting.